This invention relates to a tube bending device, especially for thin walled tubes of large diameter made from austenitic materials.
Bending of piping made from austenitic high temperature steels, e.g., steels like Cr Ni 1810 and the other steels of similar composition, in required high quality indicators, is a very complicated engineering problem, in that high or low bend angles have been realized only by a few manufacturers. The available technical literature does not contain any information about bending of large diameter, thin-walled piping with the specific thickness, i.e. with a ratio of the wall thickness to the external tube diameter less than 5%.
The tubes from austenitic materials are now bent by heating on a bending plate, then on a bending machine with a pressure bending roller, and subsequently on a bending machine with a revolving arm. The bending plate of a common bending unit is a device consisting of a rugged horizontal circular revolving platform having a diameter of several meters and two vertical guide rolls disposed off the circuit of the revolving platform. In the top face of the revolving platform, vertical holes are formed in concentric circular rows. Two shell binding pins are adjustably located in the vertical holes. A tube to be bent is closed on both ends and its internal space is filled with tamped sand. The inner surface of thin walled tubes bent as described above frequently becomes wavy and the originally circular section is flattened. With regard to bed heat conductivity of austenitic steels, the inner surface of the tube deteriorates after long-term reheating in a gas fired furnace, with burned-in sand grains and adverse material structure effects along the whole section of the wall.
The bending machine with a pressure bending roller consists of a bed with a longitudinal slidingly located pressure truck, a drive unit fixed on one end of the bed and kinematically connected to the mentioned pressure truck by a motion screw. A carrier is fixed to the second end of the bed and provided with a transversely sliding support with a pivoted bending roller and further consists of an adjustable gear with guide rollers and an induction heating coil for medium frequency electrical heating of the formed tube. When the machine is in use, the pressure bending roller bends the tube, said tube axially passing through the induction heating coil into a pressure zone where pressure is provided by a hydraulic cylinder. With regard to a quick heating zone, the surface and material structure of the bent tube have a high quality after bending, but at the bending zone, inadmissible flattening occurs which remains in the finished product. On the bending machine with a pressure bending roller, it is also very difficult to achieve an exact bend radius.
Thin-walled austenitic piping with a specific thickness less than 5% cannot be bent on a medium frequency bending machine with a revolving arm. The bending machine with a revolving arm has, in plan view, a form of a capital letter "L", where the longer leg is a main bed with a drive unit, a slidingly located pressure truck, an adjustable gear and induction coil, and where the shorter leg is an auxiliary bed with slidingly located support, carrying a revolving bending arm. On the free end of the bending arm is arranged a vise for clamping of the front end of the bent tube. The back end of the bent tube is located in the pressure truck. This machine, however, cannot provide a continuous and smooth bend; cracks and abnormal ovality occur repeatedly. The number of defects grows quickly with decreasing bend radius.
The induction coil surrounds the periphery of the tube. The distance between the inductor and the outer surface of the tube, divided by the distance between the inductor and the inner surface of the tube is set at between 0.4 and 0.9 for best results. In the foregoing ratio, the outer surface of the tube is defined, referring to the bend to be made in the tube, as the longest chordal line of the bend and the inner surface as the shortest chordal line of the bend.
In addition to the foregoing, the inductor is provided with a cooling part as well as a heating part.
According to the invention, there is provided a tube bending device comprising means for applying pressure to move the tube through the device, such as a pressure truck, an adjustable gear, an inductor coil and means for bending the tube as it exits from the inductor coil, such as a revolving arm. The angle between the heating and cooling parts of the induction coil is in the range from 3 .degree.-10.degree., the vertex of the angle .beta. being situated inside the bend of the tube.
Preferably, the plane of the inductor is offset with regard to the axis of a pin defining the rotational center of the revolving arm along the tube longitudinal axis by a distance defined by the formula EQU C=X.multidot.R/D
where coefficient X, determined experimentally by Applicant, is in the range of 0.1-4; C is the distance between the plane of the inductor and the axis of said pin, R is the mean radius of the bent section of the tube and D is the outer diameter of the tube. The shift C in location of the inductor is toward the rear of the tube.
A device according to the invention enables the fomation of a bent tube in a peripherally formed heating zone, which guarantees a continuous and smooth bend without cracking on the tension side. It is also possible to bend thin-walled piping with a specific thickness under 5%, and where it is possible to reach a fillet, the mean radius of which is equal to triple the mean diameter of the bent piping in this zone.